1. Field of the Invention
The invention relates to a d.c. voltage supply circuit for fluorescent lamps.
2. Description of Related Art
Since electronic ballasts for fluorescent lamps require a d.c. voltage supply, the available a.c. line voltage is converted into a constant d.c. voltage. In the case of rectifying circuits, energy may be only drawn from the power network if the input line voltage is higher than the output voltage. Resultant short-time current peaks bring about an inadmissible load of the power network. To comply with the respective rules, it is customary to series-connect a filter arrangement of inductivity and capacity.
Said filter arrangement is a low-pass having a limiting frequency corresponding to the network frequency. The latter being relatively low, it is necessary for the choke to have a high inductivity, thus implying, with a corresponding power consumption of the load, a big-sized choke having a high percentage of iron, large dimensions and a high weight.
To avoid using a low-pass filter with an expensive choke, it has been known to arrange in a d.c. supply circuit for fluorescent lamps a flyback converter (DE-OS 30 25 405) having a coil of a relatively low inductivity which, by a switch, is periodically connected to the output of a rectifying circuit and which, via a rectifier, is subsequently discharged onto a capacitor. In the conductive phase of the switch, the inductivity is charged and, in the off-phase of the switch, the inductivity is discharged onto the capacitor with a resultant rise of the voltage at the capacitor, i.e. of the output voltage of the flyback converter. On the other hand, the capacitor voltage is continuously discharged by the load. The frequency at which the switch is changed over is much higher than the network frequency so that the envelope of the needle-type charging current pulses has a time curve following the sinusoidal shape of the rectified line voltage. To avoid that, by the needle-shaped current pulses, high-frequency disturbances get into the supply line, it is necessary to use a relatively simple filter circuit having a low inductivy.
In case of the known d.c. voltage supply circuit, the flyback converter is an up-chopper-circuit with the inductivity in an input line, while the switch is arranged in a transverse branch between input lines. Inductivity is connected via the diode to an output line of the flyback converter. In case of a high current requirement of the load, said current may flow directly from the rectifying circuit through the series arrangement of inductivity and diode. On the other hand, there is the risk that, through said series connection, overvoltages of the supply line directly breakdown to the load and affect its constructional elements. Such overvoltages may consist of short-term voltage peaks or they may be also caused by faulty switching. An excessively high voltage to which the supply circuit may be connected, will be effective as far as to the load. Another disadvantage of the known flyback converter resides in the fact that the frequency for controlling the switch to recharge inductivity is dependent on the variation in time of the discharging current, which, on its turn, is dependent upon the difference between the output voltage and the input voltage of the flyback converter. To avoid losses, the switch should be changed to the conductive state, if the value of the discharge current of the inductivity has reached zero. The time required to this effect, is dictated by the difference between the output and input voltage. In case of a constant output voltage, said difference varies responsive to the variation in time of the input voltage which is a rectified alternating voltage.
It is the object of an invention to provide a d.c. voltage supply circuit which is applicable to a wide range of input voltages and by which the connected load, namely the fluorescent lamp choke, is protected against overvoltages.